This invention relates generally to the detection and generation of magnetic fields and, more particularly, to an apparatus for reducing the effect of an external magnetic field within a partially open shielding structure by the generation of compensating magnetic fields in the vicinity of the opening.
Many types of electronics equipment are deleteriously affected by the presence of external magnetic fields which are significantly greater than the earth's field. One example of such equipment, and the particular one to which the present invention is directed, is a color monitor having a shadow mask cathode ray tube (CRT) as the display element. A magnetic field has two major effects on the performance of a color CRT display: (1) it shifts the entire electron beam raster vertically or horizontally, or a combination of both, and (2) it causes the displayed colors to change as the beam is deflected away from its intended phosphors to ones of a different color.
There are situations, especially in tactical environments, where external magnetic fields of up to five gauss (approximately ten times the magnetic field generated by the earth) may be expected. In such environments, it is generally required that the presence of such a field will not deleteriously affect performance of the resident electronic equipment. Furthermore, it is also expected that the equipment will operate unaffected by magnetic fields within this range which are time-variable at a rate of up to 20 gauss per second.
While the effect of these magnetic field conditions has only a negligible impact on the overall position of the electron beam raster on a CRT faceplate, it has a very significant impact on the color display performance and, indeed, if color information in a video transmission is of operational significance, such information may be displayed erronously or, at best, ambiguously. It is therefore of great importance that the CRT in a color monitor be protected from the effects of external magnetic fields, and, in particular, time-varying magnetic fields.
A commonly-used practice for protecting electronic equipment from the effects of an external magnetic field is to enclose the apparatus within a shield fabricated of a high permeability material. The shield serves to attract the magnetic flux lines toward itself, and provides a magnetically-conductive path, directing the flux around and away from the apparatus inside the shield. However, it is easily recognized that a CRT cannot be totally enclosed within a shield structure; clearly, there cannot be a magnetic shield in front of the CRT faceplate. Locating the CRT within an open shielding structure will reduce the effects of a magnetic field, especially if the field has significant components perpendicular to the main axis of the tube, i.e., the axis extending through the center of the faceplate and the gun at the rear of the neck. However, since shielding cannot be placed across the faceplate, the monitor must be placed in an active enclosure which is able to counteract magnetic fields having significant components parallel to the main axis of the CRT.
The beneficial effects of shielding are enhanced, even for substantially axial magnetic fields, by recessing the CRT deeply within a shielded enclosure. However, this practice reduces the angle from which the monitor may be viewed. Furthermore, the additional space needed by an enclosure which would provide an adequate recess for the CRT may not be available, and space considerations may dictate that the CRT be positioned such that its faceplate is positioned in proximity to the opening in the shielding enclosure.
It is therefore necessary to provide a means for reducing the effects of an external magnetic field in the vicinity of the opening of a partially open magnetic shielding structure. It is also necessary to provide such magnetic field reducing means which is efficacious in response to a time-varying external magnetic field.